1. Field of the Invention
The invention relates to apparatus and methods for forming plastic articles and, more particularly, to apparatus and methods employing principles of blow molding.
2. Background Art
Blow molding technology associated with the forming of relatively hollow, plastic articles is often referred to as “extrusion blow molding.” Basic principles of extrusion blow molding technology for making hollow thermoplastic shapes are described in the Modern Plastics Encyclopedia 1979, 1980, beginning on page 230. The text of this publication is incorporated herein by reference.
In brief summary, known extrusion blow molding technology for forming thermoplastic articles utilizes molten plastic which is extruded downwardly within a blow molding apparatus, in the form of a parison. A “parison” can be characterized as being typically cylindrical in shape and formed of plastic. Once the plastic has been extruded into the form of the parison, the blow molding apparatus can close a mold around the parison. When the mold has been appropriately closed, air can be injected and forced into the parison, which is characterized as “blow molding” plastic. As the air is being forced inside of the parison, the cavity formed by the parison in the mold is pressurized. The molten plastic is therefore forced against the outer walls of the mold.
When the plastic article to be formed is sufficiently cooled, the article can be ejected from the mold. It should be noted that with this process, the inner wall of the article (formed inside of the parison) is formed only by the pressurization of the cavity. This inner wall can therefore vary relatively significantly by either the thickness of the parison, or the shape of the article. Accordingly, known blow molding apparatus and methods associated therewith can have certain disadvantages when customers for the plastic articles require relatively tight tolerances on the inner walls of the plastic articles.
As previously stated, various apparatus and methods have been developed relating to concepts associated with blow molding plastic articles. To overcome the problem of providing for relatively tight tolerances for inner walls of the blow molded articles, it is known to use a blow molding technique referred to as “bottom blow” technology. This and other blow molding apparatus and methods known in the prior art are described in the following references.
More specifically, Maass, U.S. Pat. No. 3,048,891, issued Aug. 14, 1962, discloses a blow molding machine and associated process for forming hollow articles from organic plastic material. In particular, the Maass patent discloses the concept of insertion of coring pins 66 into the ends of a parison, before entry of a blow pin or a needle 74 into the side. This function is performed so as to form a final article having a walled core structure.
Peters, U.S. Pat. No. 3,973,896, issued Jan. 24, 1989, discloses the concept of a blow molding apparatus with a mandrel carried by portions of the mold, and movable between extended or retracted positions. The mandrel has a free end with a vacuum surface area portion. The free end is engageable with the parison when the mandrel is in the extended position. A blow needle is reciprocally mounted in the mandrel. Insertion of the needle into the parison is facilitated by the parison being held against the end of the mandrel by the vacuum.
Troutman, et al, U.S. Pat. No. 5,078,948, issued Jan. 7, 1992, discloses a particular blow needle structure having a tubular portion and a cutting tip atop the tubular portion. The cutting tip includes three or more blades terminating in a point. The tubular portion includes passages which communicate with the source of pressurized fluid. The “arrow head” needle penetrates the parison and forms “flap sections” in the parison. The needle tip pushes the flap sections inwardly within the parison.
Omi et al, U.S. Pat. No. 6,537,056, issued Mar. 25, 2003 is also directed to a specific structure for a needle blow nozzle. The needle blow nozzle includes a tubular body with a needle portion, needle center, slopes and nozzle holes. The needle is laterally inserted into a parison for jetting air into the interior of the parison. The slopes are formed so as to extend from the needle center toward an outer peripheral surface of the tubular body. In this manner, edges are provided which extend at an acute angle, and the nozzle holes are formed in the needle portion.
Other patent references found in our search are directed to several other concepts associated with blow molding assemblies and processes. For example, Hahn, U.S. Pat. No. 4,028,034, issued Jun. 7, 1977, is directed to a method and apparatus for blow molding plastic, where the mold has a quadrasectional configuration. Particles are extruded from an extrusion unit 10 as a tubular configuration 16 for introduction within the mold 18. The article is blow-molded using an expanded fluid introduced through a hollow needle 32 at the side of the mold. The article is severed into two portions through the use of a hot wire.
Roggenburg, Jr., et al, U.S. Pat. No. 4,332,750, issued Jun. 1, 1982, discloses a blow molding apparatus having a gate which connects the blow molded hollow shapes to the parison. Subsequent degating of the shapes is provided by ultrasonic mechanical vibrations applied to the gates.
Peters, U.S. Pat. No. 4,799,876, issued Jan. 24, 1989, is primarily directed to a blow needle configuration. The needle includes a sealing member which surrounds the blow needle. During inflation of the parison, projections come into sealing contact with the exterior surface. This concept is disclosed as preventing leakage of blow gas from the interior of the parison around the blow needle.
Raboin, U.S. Pat. No. 6,090,339, issued Jul. 18, 2000, is directed to a process for manufacture of a blow-molded plastic extruded product having a specific configuration. The blow mold apparatus includes a pair of mold sections which are closed along a linear direction of travel. When closed, they form a mold cavity which includes a narrow section extending in the direction of travel. This section has a width which is less than approximately twice a wall thickness of the parison. As the mold sections close, they sever and enclose a segment of the parison within the mold cavity, and shear a double-wall section of the parison into the narrow section. This forms a reinforcing flange on the resultant product.
Hutchins, U.S. Pat. No. 6,440,353, issued Aug. 27, 2002, is directed expressly to a vacuum forming method for twin-sheet vacuum forming parts in a vertical orientation. With twin or dual-sheet forming, two sheets of plastic sheet are parallel-laid in a horizontal fashion, and heated to a predetermined temperature. In the Hutchins method, two parallel sheets of plastic synthetic resin are clamped together in a vertical orientation to a clamping tray. The sheets directly contact each other about the perimeter of the tray. Air is then blown between the sheets so as to create the appropriate hollow cavity, and the sheets are heated to molding temperatures in a vertical orientation. A vertically-oriented split mold is then opened, with the mold cavities having a contour which conforms with configuration of the desired article. The two parallel sheets are then disposed between the opposing mold cavities of the split mold, and negative pressure is applied to vacuum section holes provided in the mold cavities. The split mold is then closed so as to form a “pinch off” around the perimeter of the hollow cavity. In this manner, a molded hollow part is formed which is still disposed within the clamping tray. A blow pin is then inserted into the hollow cavity for blowing compressed air into the cavity to push the parallel sheets against the mold cavities. The molded part is then cooled to a set point, the vertically-oriented split mold is opened, and the clamping tray withdrawn.
Ohno, et al, U.S. Patent Application Publication No. 2006/0204606, published Sep. 14, 2006, is directed to cooling processes associated with blow molding methods and apparatus. More specifically, upper and lower portions of a parison are forcibly cooled, using upper and lower holes of respective needle blow nozzles. The metallic mold is cooled specifically by a cooling apparatus within the mold. The upper clamp portion and lower clamp portion formed on the upper and lower portions of the parison, respectively, are cooled by air which passes only through an upper discharge hole formed on the upper needle blow nozzle and a lower discharge hole formed on the lower needle blow nozzle.
Nemoto, U.S. Patent Application Publication No. 2008/0038497, published Feb. 14, 2008, is directed to a blow molding die assembly and associated methods for manufacture of resin hollow bodies. One of the die assemblies comprises a pair of main dies which are formed with a dividable cavity, by closing open end surfaces with each other. A pair of slide dies is disposed outside of the main pair of dies, and have abutting portions that move between first positions where the abutting portions project outwardly from the open end surfaces, and second positions where the abutting portions are located inward of the open end surfaces of the main dies. A method for manufacturing resin hollow bodies uses the aforementioned die assembly with a series of opening and closing steps.
As earlier stated, problems exist with respect to a number of the prior art apparatus and methods associated with blow molding technology when the formed article requires relatively tight or small tolerances with respect to the thickness of the parison or the shape of the article itself. For example, and as earlier stated, it is known to employ what is characterized as a “bottom blow” process which provides at least some improvement over other known processes for generating blow molded plastic articles having relatively tight tolerances on the inner walls of the articles. Assuming that the article to be formed includes a neck portion, the bottom blow process involves dropping the parison over a machine and blow pin. The blow pin remains inside the mold, and forms the inner diameter of the neck of the article. Such a process could possibly be applied to blow molded articles such as necks of engine resonators. Such engine resonators utilize what is often referred to as “Helmholtz resonance,” which is essentially the phenomenon of air resonance within a cavity. Airboxes and other components of internal combustion engines use the principles of Helmholtz resonance for sound muffling and other purposes. In the fabrication of engine resonators, the dimensions of the necks of the resonators are of particular importance in proper operation. Accordingly, tight tolerances are required on the “inner diameter” or inside wall of the resonator necks. In fact, tolerances on the order of +/−0.5 millimeters may be considered. However, with bottom blow processes, it is often difficult to accurately form a desired length and shape of various types of resonators and other blow molded articles. In particular, for certain shapes and article lengths, the required parisons for such articles to be bottom blown would be extremely difficult to program into a molding apparatus.